Exhibit 10.6.6

FIFTH AMENDMENT OF VENTURE LOAN AND SECURITY AGREEMENT

This FIFTH AMENDMENT OF VENTURE LOAN AND SECURITY AGREEMENT (this “Agreement”), effective and dated as of November 1, 2013, is entered into by and between Xtera Communications, Inc., a Delaware corporation (“Xtera” or “Borrower”) and Horizon Funding Trust 2013-1 (“Horizon Trust”), as assignee of Horizon Technology Finance Corporation, a Delaware corporation (“Horizon” and collectively with Horizon Trust, “Lender”).

RECITALS

A. Xtera and Lender are parties to a certain Venture Loan and Security Agreement dated as of May 10, 2011, as amended from time to time (as amended, the “Loan Agreement”) pursuant to which Lender, among other things, has (i) provided certain loans to Xtera as evidenced by (1) a certain Secured Promissory Note (Loan A) executed by Xtera in favor of Lender, dated as of May 10, 2011, in the original principal amount of Ten Million Dollars ($10,000,000) (the “Loan A Note”) and (2) a certain Secured Promissory Note (Loan B) executed by Xtera in favor of Lender, dated as of December 27, 2011, in the original principal amount of Two Million Dollars ($2,000,000) (the “Loan B Note” and together with the Loan A Note, the “Notes”), and (ii) been granted a security interest in all assets of Xtera, including Intellectual Property (as defined in the Loan Agreement).

B. Horizon transferred all right, title and interest in and to the Notes to Horizon Funding 2013-1 LLC (“Funding”) on or about June 28, 2013.

C. Funding subsequently sold all right, title and interest in and to the Notes to Horizon Trust on or about June 28, 2013.

D. Xtera has now requested that Lender amend the Loan Agreement to revise the repayment schedule with respect to the Notes.

E. Lender is willing to grant such request, but only to the extent, and in accordance with the terms, and subject to the conditions, set forth herein.

AGREEMENT

NOW, THEREFORE, in consideration of the above recitals and for other good and valuable consideration, the receipt and adequacy of which are hereby acknowledged, Co-Borrowers and Lender hereby agree as follows:

““Loan A Maturity Date” means July 1, 2015, or if earlier, the date of acceleration of all Obligations outstanding with respect to Loan A following an Event of Default or the date of prepayment, whichever is applicable.”

““Loan B Maturity Date” means February 1, 2016, or if earlier, the date of acceleration of all Obligations outstanding with respect to Loan B following an Event of Default or the date of prepayment, whichever is applicable.”

“Late Payment Fee. If any Scheduled Payment due from Borrower on or before February 1, 2014 is not made when due hereunder, Borrower shall, in addition to making such Scheduled Payment, pay to Lender a late payment fee equal to six percent (6%) of such Scheduled Payment. If any Scheduled Payment due from Borrower on or after March 1, 2014 is not made when due hereunder. Borrower shall, in addition to making such Scheduled Payment pay to Lender (i) if such Scheduled Payment is made prior to the first calendar day of the month immediately following the calendar month in which such Scheduled Payment is due, a late payment fee equal to six percent (6%) of such Scheduled Payment, or (ii) if such Scheduled Payment is made on or after the first calendar day of the month immediately following the calendar month in which such Scheduled Payment is due, a late payment fee equal to the lesser of (i) One Hundred Thousand Dollars ($100,000) or (ii) the product of (a) 0.5 multiplied by (b) the total amount of the Scheduled Payment that was not made when due.”

“Optional Prepayment. Upon ten (10) Business Days’ prior written notice to Lender, Borrower may, at its option, at any time, prepay all, and not less than all, of the Loans in full by paying to Lender an amount equal to (i) all accrued and unpaid Scheduled Payments with respect to the Loans due prior to the date of prepayment; (ii) any accrued and unpaid interest on the outstanding principal amount of the Loans, (iii) an amount equal to (A) if the Loans are prepaid within twenty-two (22) months from the Funding Date, four (4%) percent of the then outstanding principal balance of the Loans, (B) if the Loans are prepaid more than twenty-two (22) months from the Funding Date but less than thirty-four (34) months from the Funding Date, three (3%) percent of the then outstanding principal balance of the Loans, or (C) if the Loans are prepaid more than

- 2 -

thirty-four (34) months from the Funding Date but prior to the Maturity Date, one (1%) percent of the then outstanding principal balance of the Loans, (iv) the outstanding principal balance of the Loans, and (iv) all other sums, if any, that shall have become due and payable hereunder. Except as set forth in this Section 2.3, the Loans may not be prepaid.”

“Loan A Final Payment. Borrower shall pay to Lender a payment in the amount of One Million Four Hundred Seventy-Seven Thousand Forty-One and 31/100 Dollars ($1,477,041.31) (the “Loan A Final Payment”) upon the earlier of (i) payment in full of the principal balance of Loan A, (ii) an Event of Default and demand by Lender of payment in full of Loan A or (iii) on the Loan A Maturity Date, as applicable.”

“Loan B Final Payment. Borrower shall pay to Lender a payment in the amount of Two Hundred Seventy-Two Thousand Nine Hundred Fifty-Eight and 44/100 Dollars ($272,958.44) (the “Loan B Final Payment”) upon the earlier of (i) payment in full of the principal balance of Loan B, (ii) an Event of Default and demand by Lender of payment in full of Loan B or (iii) on the Loan B Maturity Date, as applicable.”

- 3 -

[Remainder of page intentionally blank. Signature page follows]

- 4 -

IN WITNESS WHEREOF, Borrower and Lender have caused this Agreement to be executed as of the day and year first above written.

- 5 -